The present invention relates to the method used to recover rare earth elements and, in particular, chemical compounds of rare earth elements from phosphogypsum.
In today's rare earth industry more than 60% of expenses are connected with ore mining and processing. Therefore, it is profitable to use technogenic primary products, where rare earth elements are the minor components being mined from subsurface mineral resources. Apatite phosphogypsum can be used as primary product for such purposes. Phosphogypsum is produced during sulfuric-acid processing of apatite concentrate for manufacturing mineral fertilizers. Phosphogypsum is a product of technogenic origin in contrast to natural gypsum stone consisting of CaSO4×2H2O. That is the reason why the latter has a considerable amount of impurities which are compounds of rare earth and other elements (8 -19%). The content of rare earth elements is within 0.5-1.1% depending on the composition of apatite concentrate.
There is a known process which relates to the recovery of rare earth elements from phosphogypsum (see RU Patent N 2225892 MPK C22B 59/00) and involves processing of phosphogypsum using 25% sulfuric acid solution. The rate of recovery of rare earth elements is 50.0-60.2%. The time of such sulfuric-acid processing is 3 hours; sulfuric acid concentration is 20-25% by weight with liquids-solids ratio (L:S) equal to 2-3. Crystallization of rare earth elements is carried out by inoculating sulfites of rare earth elements at solids-liquids ratio exceeding 100. Such technology requires that the production involves a large quantity of processing equipment. Besides, the disadvantages of this method include low recovery rate of rare earth elements, a considerable number of processing operations, large volumes of cycling sulfuric acid solutions and time expenditures.
There is another known method (RU Patent N 2337897, MPK C 01F 11/46) which involves extraction of compounds of rare earth elements and phosphor into solution with formation of insoluble gypsum residue containing large quantities of sulfuric and phosphoric acids within 20-25 minutes using processing by 22-30% sulfuric acid solution. The extraction solution also contains double sulfates of rare earth elements with sodium or potassium. The resulted crystalline gypsum is subject to processing by Ca(OH)2.CaO or CaCO3 in order to deacidify residues of sulfuric and phosphoric acids unless their pH-value exceeds 5. At the same time it is required to control the content of phosphor impurities in spent liquor and, depending on the ratio of their content to residual gypsum humidity, to subject this spent liquor to extraction or purification processes using TiOSO4H2O until permissible P2O5 content is achieved.
This method allows to accomplish extraction of up to 82.1% of rare earth elements into an extraction solution with their further separation by crystallization into concentrate with recovery rate of up to 68.5%.
The disadvantages of this method are the pH-value of the resulted crystalline gypsum which is not in compliance with permissible construction standards as well as high concentration of P2O5. Separation of lanthanoid elements from the oversaturated extraction solution is rather a time-consuming process (2 hours). Both the content of phosphor impurities in acid extraction agent and residual humidity of gypsum residue should be controlled. In order to remove excess phosphor, special equipment is required for deacidification of phosphorous compounds using dry titanium compounds or a mixture of titanium and concentrated sulfuric acid with further separation of titanyl phosphate and obligatory processing using concentrated sulfuric acid. In case concentration of sulfuric acid in extraction solution is reduced below 22%, sulfuric acid concentration should be recovered in order it can be used again in the process. This will require a large quantity of reactor, capacitive and filtration equipment to be used for extraction of rare earth elements, storage of different extraction solutions and deacidification of phosphorous compounds.
There is a known nitric-acid technology for recovery of rare earth elements from apatite with 85% separation into the solution which also contains phosphor and fluorine (V. D. Kosynkin et al. “State of the Art and Development Perspectives of Rare-Earth Industry in Russia”—“Metals”, N 1, 2001).
The most similar to the claimed method is a method for recovering rare earth elements from phosphogypsum (see RF Patent N 2293781, MPK C22B 59/00) which involves processing of phosphogypsum by sulfuric acid with further extraction of rare earth elements into solution, separation of insoluble gypsum residue, increasing content of rare earth elements in oversaturated solution in order to crystallize the concentrate of rare earth elements, separation of the concentrate from spent liquor and its processing. The phosphogypsum is processed using sulfuric acid of 22-30% (by weight) concentration at liquids-solids ratio of 1.8-2.2 within 20-30 minutes in order to avoid spontaneous crystallization of rare earth elements from the solution prior to separation of the insoluble residue. Oversaturation of the solution is achieved due to sodium content of 0.4-1.2 g/l.
The disadvantage of this known method are the need in additional reagents, high concentrations of acids together with their considerable volumes, a large number of basic technological operations with an insufficient rate of recovery of rare earth elements and total complexity of the method.